1. Field of the Invention
The present invention relates to a semiconductor optical modulator for use in an optical communication system requiring a high speed operation, and in particular to a semiconductor optical modulator having a function of guiding and releasing carriers accumulated in a light absorption layer by applying excitation light and a semiconductor optical device having such a semiconductor optical modulator and a semiconductor laser integrated on the same substrate.
2. Description of the Related Art
In recent years, enormous volumes of data in communications have been transmitted through high-performance information and communication instruments so that it becomes essential to popularize widespread optical communication networks using optical fibers. In the optical communication networks, high-speed semiconductor lasers or the like are used as key devices thereof, and semiconductor optical modulators are also used for modulating input light beams generated by the semiconductor lasers. Hereinafter, an assembly structure of a combination of semiconductor optical elements, including such as a semiconductor laser and a semiconductor optical modulator, integrated on the same substrate is called a xe2x80x9csemiconductor optical device.xe2x80x9d
Generally, in the intrinsic absorption of light in a semiconductor, electrons and holes are created from photons when the photon energy hxcexd is greater than the bandgap Eg. The intrinsic absorption edge corresponding to the long-wavelength side Eg of the intrinsic absorption band can be shifted toward a longer wavelength by application of a high electric field to the semiconductor, which is called xe2x80x9cFranz-Keldysh effectxe2x80x9d.
In an optical modulator using a semiconductor material, an absorption coefficient or refraction index can be significantly changed by the Franz-Keldysh effect or quantum confined Stark effect. In this case, each optical modulator shares the same type of material with each light-emitting device so that it can be integrated into a small high-efficiency modulator for external light. In addition, such an optical modulator has achieved a high speed operation in a certain modulation frequency band to a degree as achieved by a dielectric optical modulator.
In an electroabsorption type optical modulator, the amount of carriers comprised of pairs of electrons and holes (referred to as xe2x80x9celectron and hole pair(s)xe2x80x9d, hereinafter) generated by light absorption increases in accordance with incident light intensity. The electron and hole pairs form an internal electric field so as to cancel an externally applied electric field. The screening effect on the externally applied electric field increases with the intensity level of the incident light, and there is a correlation between the intensity level of the incident light and the change in the absorption coefficient.
FIG. 14 is a schematic view showing a conventional electroabsorption type semiconductor optical modulator 100. In FIG. 14, reference numeral 101 represents an n-conductivity type InP substrate, 102 an InGaAsP light absorption layer, 103 a p-conductivity type InP cladding layer, 104 a p-conductivity type InGaAsP contact layer, 105 a SiO2 insulating film, 106 a Ti/Au anode electrode, and 107 a Ti/Au cathode electrode. FIG. 15 is a schematic model showing an energy band of a portion near the light absorption layer 102. In FIG. 15, the bandgap energy is represented by E1 and the corresponding bandgap wavelength is represented by xcex1 (xcex1=hc/E1).
Referring to FIGS. 14 and 15, the operation of the conventional semiconductor optical modulator 100 is described below. In the optical modulator 100, continuous wavelength light (hereinafter abbreviated as xe2x80x9cCW lightxe2x80x9d) is used as a high-intensity incident light beam Lin having a wavelength of xcex1, which is inputted into one facet, and a modulated output light beam Lout (xcex1 in wavelength) is outputted from the other opposed facet. At the same time, an external voltage is applied in the inverse direction between the anode electrode 106 and the cathode electrode 107 in the optical modulator 100. As shown in FIG. 15, the externally applied voltage causes the Franz-Keldysh effect by which the effective bandgap energy E1 of the light absorption layer 102 is reduced and the absorption coefficient with respect to longer wavelengths than the bandgap wavelength is increased. This change in absorption coefficient by the voltage application is used for the modulation of the light intensity.
In the conventional semiconductor optical modulator 100, however, when the incident light beam has an intensity of 20 mW or more, the carriers formed of the electron and hole pairs are accumulated in a well part of the light absorption layer 102. Such accumulated carriers screen and attenuate the electric field applied to the light absorption layer 102 according to Gauss"" law.
In order to reduce the carriers accumulated in the light absorption layer 102, a reverse bias voltage is applied between the anode electrode 106 and-the cathode electrode 107 so that the carriers drift toward the p-InP cladding layer 103 and the n-InP substrate 101. Such carrier release, however, needs a relaxation time of about 1 ns, leading to a relatively low response speed. Thus, the conventional device involves problems of a lower extinction ratio at high frequency, a distorted light waveform, and deteriorated transmission characteristics in the optical communication system that require a high speed performance of 40 Gbit/s or so.
The present invention has been made in order to solve the above-mentioned problems. It is an object of the present invention to provide a semiconductor optical modulator which can guide and release the accumulated carriers at high speed, respond at a high frequency of 100 GHz or more, preventing deterioration of an extinction ratio, and having a high resistance to an input light as well as good transmission characteristics, even when an incident light beam having an intensity of 20 mW or more is inputted.
Another object of the present invention is to provide a semiconductor optical device including the above-mentioned semiconductor optical modulator which is monolithically integrated on the same substrate.
In order to achieve the above-mentioned objects, the present invention provides a semiconductor optical modulator of an electroabsorption type for modulating an incident light beam by use of changes in absorption coefficient under application of an external voltage. The semiconductor optical modulator comprises: an input facet for receiving the incident light beam of a first wavelength to be modulated; an output facet, which is opposed to the input facet, for outputting a modulated light beam; a light absorption layer, which is formed on a semiconductor substrate, for absorbing the incident light beam and thereby generating carriers; a carrier accumulation portion for accumulating the generated carriers; and a carrier guide and release portion having a well layer formed in the light absorption layer, for guiding and releasing the accumulated carriers outside.
The carrier guide and release portion guides and releases the carriers accumulated in the well layer upon receipt of an incident excitation light beam, of a second wavelength which corresponds to bandgap energy of the well layer.
In this configuration, the carriers are guided and released through the well layer with receipt of the excitation light beam, which allows the semiconductor optical modulator to guide and release the accumulated carriers at high speed, to respond at high frequency, preventing deterioration of an extinction ratio, with good transmission characteristics, even when an incident light beam having an intensity of 20 mW or more is input.
According to another aspect of the present invention, a semiconductor optical device includes the semiconductor optical modulator having the above-mentioned basic structure and an excitation light generating semiconductor laser for generating the excitation light beam, wherein both of the optical modulator and semiconductor laser are integrated on the same substrate.
By this arrangement, the light source for generating the excitation light beam and the optical modulator are monolithically integrated on the same substrate so that the semiconductor optical device can be reduced in size and manufacturing cost.